Rotary potentiometer with means for limiting rotation of the operating shaft thereof



A ril 5, 1960 ROTARY POTE ROTATION OF THE Filed J. M. LOONEY. JNTIOME'I'ER WI OPER March 3, 1

MEAN 01 2 LIMITING NG FT THEREOF INVENTOR.

Imrmy/z 100m; J'A. BY fizz/#2244 United States Patent ROTARYPOTENTIOMETER WITH MEANS FOR LIMITING ROTATION OF THE OPERATING SHAFTTHEREOF Joseph M. Looney, Jr., Tarzana, Calif., assignor to ActonLaboratories, Inc., Acton, Mass., a corporation of MassachusettsApplication March 3, 1959, Serial No. 796,843

3 Claims. (Cl. 338-135) This invention relates to potentiometers andmore particularly to improved means for limiting rotation of theoperating shafts of rotary potentiometers.

In rotary potentiometers it is often necessary to provide stop means forlimiting rotation of the operating shaft to an angle of less than 360degrees. To be effective, the stop means must be of relatively sturdyconstruction. This is particularly true in trimming potenti ometerswhere friction means are provided to apply a braking action on theoperating shaft, whereby a relatively high amount of torque is requiredto be applied to turn the shaft to vary the setting of the slider. Insuch a case, it is sometimes difficult for the operator to discriminatebetween the drag or braking force applied to the shaft by the frictionmeans and the resistance to rotation offered by the stop mechanism; andin his enthusiasm to rotate the shaft further, the operator mayunintentionally break the stop mechanism. While making the stopmechanism stronger will obviate such an accident, it is difiicult inmany cases to accomplish this without making the stop mechanism larger.This is an unsatisfactory solution in the case of small precisionpotentiometers designed for use in airborne equipment where size andweight are critical factors.

Accordingly, the primary object of this invention is to provide a smallbut exceptionally sturdy stop mechanism for limiting rotation of theoperating shaft of rotary potentiometers.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

Fig. 1 is a vertical section through a potentiometer embodying thepresent invention;

Fig. 2 is a cross-sectional view taken along line 2-2 of Fig. l; and

Fig. 3 is a cross-sectional view taken along line 33 of Fig. 1.

Referring now to Fig. 1, there is shown a rotary potentiometercomprising a cylindrical casing 2 having a removable wall 4 secured toone end thereof and a fixed wall 6 at the other end thereof. Aconventional wound wire resistance element 8 extends around and isadhesively secured to the interior surface of the casing. Resistanceelement 8 extends through an angle slightly less than 360 degrees.

Supported by a bushing 10 which is secured by means of a force fit in acentrally located hole 12 in end wall 6 is an operating shaft 14. Theinner end of shaft 14 extends into casing 2 and carries a split collar16 which has a radially extending arm 18. Attached to the free end ofarm 18 is a resilient contact member 20 which slidingly engagesresistance element 8. Contact member 20 is formed of electricallyconductive spring metal. Although not shown, it is to be understood thatcasing 2 is provided with two input terminals and one output terminal,means electrically connecting the two input terminals to the oppositeends of resistance element 8, a contact ring, means electricallyconnecting the contact ring to the input terminals, and meanselectrically connecting contact member 20 to the contact ring, whereby avoltage may be applied across resistance element 8 and an output voltagederived by means of contact member 20. Such elements are old as shown,for example, by US. Patent No. 2,778,906, issued January 22, 1957, to L.M. Burgess, and are omitted from the drawings solely to better emphasizethe novel features of the present invention.

Bushing 10 consists of three integral cylindrical sections 22, 24, and26 of different inside diameters. Bushing section 24 has the smallestinside diameter-just large enough to accommodate and rotatably supportshaft 14. Bushing section 22 has an annular spacer 28 which surroundsshaft 14 and abuts against bushing section 24. Disposed Within bushingsection 22 are two flat spring metal elements 30 and 32. The length ofspring elements 30 and 32 is greater than a chord of the circle outlinedby the inner surface of bushing section 22 that is tangent to thecircular outline of shaft 14. These spring elements are forced intobushing section 22 on opposite sides of shaft 14. Because of theirlength, these spring elements must be bent in order to fit within thebushing. The ends of these spring elements are held in tight engagementwith the inner surface of bushing portion 22 by the wedging forceexerted on them intermediate their ends by shaft 14. The spring elementsin turn exert a clamping force on the shaft, holding it againstrotation. The spring elements are held in place by an annular plug 34which surrounds shaft 14 and is force fitted into the end of bushingportion 22. An O-ring 36 mounted on shaft 14 bears against plug 34 toprevent axial movement of the shaft away from the casing 2.

To the extent already described, the device of Fig. 1 is a high-torquepotentiometer and the shaft 14 may be rotated through an angle greaterthan 360 degrees. The novel improvement hereinafter described functionsto limit rotation of shaft 14 to an angle less than 360 degrees.

Beyond bushing portion 24, shaft 14 is of stepped construction andcomprises a first or intermediate enlarged portion 40 and a secondenarged portion 42. Portion 40 has a diameter greater than the insidediameter of bushing portion 24 and smaller than the correspondingdiameter of bushing portion 26. Portion 42 has a greater diameter thanshaft portion 40. The latter abuts bushing section 24 to limit movementof shaft 14 toward casing 2.

The inner surface of bushing portion 26 has a semicylindrical groove 44which is in alignment with and forms a continuation of a small bore 46formed in bushing portion 24. A sturdy metal pin 48 is force fitted inbore 46 and is coextensive with groove 44. The outer surface of shaftportion 40 has a semi-cylindrical groove 50 which is in alignment withand forms an extension of a small bore 52 formed in shaft portion 42. Asecond sturdy metal pin 54 is seated in bore 52 and is substantiallycoextensive with groove 50. Pins 48 and 54 are force fitted in bores 46and 52. Pin 48 projects inwardly and pin 54 projects outwardly bysufficient amounts to engage each other as shown in Fig. 2 when shaft 14is rotated. It is to be noted that these stop pins make a line contactand not simply a point contact with each other. Since these pins aresupported by the surrounding surfaces formed by grooves 44 and 50, theyform a relatively strong stop mechanism to limit rotation of shaft 14.

It is to be noted that although grooves 44 and 50 have been described assemi-cylindrical in cross section, they may and preferably shouldencompass an angle slightly greater than 180 degrees so as to offer moresupport to the pins 48 and 54.

;- A primary advantageof the stop mechanism, just deil's'cr ibed'is'that it is simple and easyv to construct andi'does not, entail anincrease in the size of the potentiometer.

Another important advantage is that although the pins irerelativelysmall in size, they offer by virtue of line ontactwith eachother a relatively strong resistance not heretofore obtainable with theothcr'types of mechanical stops" except by the use of stop members jofgreater 'si ze. 'Obvio,usly,' many modificationsand variations of the 1present invention are possible in thelight of the 'above teachings.Therefore, it is to be understood that the invention isv not limited inits application to the details lfof constructionand arrangement of partsspecifically described or illustrated, and that within the scopeoof the{appended claims, it, may be practicedotherwise than as specificallydescribed or illustrated.

I claim: 7 V v v 1. In a rotary potentiometer comprising a casing, an

operating shaft extending into said casing, and a bushing attached tosaid casing having means rotatably support- .ing said shaft andacylindrical portion surrounding said shaft in spaced relationtherewith, the improvement comprising a first pin oriented in parallelrelation to said shaft and seated throughout its length in depressionormedjin said cylindrical portionlof said bushing, a secfftiori" of saidfirst pin for asubstantial portion of its length protruding into thespace between said shaft and ..said cylindrical portion in exposed andspaced relation 1 to said shaft, a second pin oriented in parallel andaxially ,Dverlapping relation to said, first pin, said secondpin seatedthroughout its length in a depression formed I in said shaft, a sectionof said second pin for a substantial portion of its length exposed tosaid bushing and protruding intdsaid spac'e'by stamens:sufficient'td'engage fin a; second-inner cylindrical Vsaidfirst'surfaceincoaxial space relatl on with said shaft,

said exposed section of said first pin when said shaft is rotatedrelative to said bushing.

2. The combination of a shaft having an outer cylindrical surface, abushinghaving a first inner cylindrical surface surrounding androtatably supporting said shaft rfac'spaced'axially' from a first pindisposed in parallel fela tion to said' 'shaft and seated'in' adaxiallyextendingf depression formed in said second "cylindrical'su'rface, 'a'portion of 'said' first pin 3. In a potentiometer comprising a casing, aresistance element disposed insa'idjcasing, a shaftextending into saidcasing, and a contact element carried by said shaft in slidable contactwith'saidre'sistance element, the iniprovement comprising ajbushing'attached to said casing and rotatably supporting said shaft, saidbushing having a cylindrical portiontn cciagiats'paeed relation withsaid shaft, a first piii]mounted;in adep'rfcs'sionformed in the innersurface or said .ljcylindr'ical fportion and having a section thereofprotruding inwardlyawayfrom said inner surface, and a'second-fpinniounted in a depression formed in saidishaft' and pr 'otrudingoutwardly of the inner surface thereof. bylan amount sufficient toengage said first pin when said'shaft is rotated'r'elative to saidbushing. No references cited.

